1. Field of the Invention
The present invention generally relates to an image forming apparatus, and more particularly to an image forming apparatus having a recording head discharging liquid droplets.
2. Description of the Related Art
As an image forming apparatus such as a printer, a facsimile machine, a copier, a multi function peripheral thereof and the like, there has been known an inkjet recording apparatus and the like employing a liquid discharging recording method using a recording head that discharges ink droplets. In the image forming apparatus employing the liquid discharging recording method, an image is formed by discharging ink droplets from a recording head onto a fed sheet. Herein, the term “forming” is a synonym of the terms recording typing, imaging, and printing. The image forming apparatus employing the liquid discharging recording method includes a serial-type image forming apparatus and a line-type image forming apparatus. In the serial-type image forming apparatus, an image is formed by discharging ink droplets from the recording head while the recording head moves in the main scanning direction. On the other hand, in the line-type image forming apparatus, an image is formed by discharging ink droplets from the line-type recording head while the recording head does not change its position.
Herein, the term “image forming apparatus” refers to an apparatus (including a simple liquid discharging apparatus) forming an image by discharging ink onto a medium including paper, thread, fiber, textile, leather, metal, plastic, glass, wood, ceramic and the like. Further, this term “image forming apparatus” refers to a simple liquid discharging apparatus as well. The term “image forming” refers to not only forming a meaningful image such as characters, figures, and the like on a medium but also forming a meaningless image such as a pattern and the like on a medium (including simply discharging droplets onto a medium by an apparatus such as so-called a droplet discharging apparatus or a liquid discharging apparatus). Further, the term “ink” is collectively used to refer to not only any material called “ink” but also any liquid for forming an image which may be called recording liquid, fixing processing liquid, liquid, a DNA sample, a patterning material or the like. Further, the term “sheet” is not limited to a material made of paper, and is collectively used to refer to any material called a medium to be recorded, a recording medium, recording paper, recording sheet, and the like to which ink (ink droplets) is adhered, the material including an OHP sheet, fabric and the like.
As a liquid discharging head (droplet discharging head) to be used as the recording head, there have been known a piezoelectric type head and a thermal-type head. In the piezoelectric type head, liquid droplets are discharged by increasing the pressure by changing a volume in the liquid chamber by displacing a vibration plate using a piezoelectric actuator or the like. On the other hand, in the thermal-type head, the liquid droplets are discharged by increasing the pressure in the liquid chamber by generating bubbles by heating a heating element in the liquid chamber by supplying a current to the heating element.
Regarding the image forming apparatus employing such a liquid discharging method, there has been a demand for the increase of the image forming speed. To that end, a method is widely used in which ink is supplied from the ink cartridge (main tank) to a sub tank (which may also be called a head tank or a buffer tank) via a tube, the ink cartridge (main tank) having a large capacity and being installed to be fixed to the apparatus body, the sub tank being disposed on the recording head. By using this method (tube-supply method) using the tube to supply ink, it becomes possible to reduce the size and weight of the carriage section, thereby enabling greatly reducing the size of the structure and driving mechanism of the apparatus.
In the tube-supply method, the ink to be consumed by the recording head for forming an image is supplied from the ink cartridge to the recording head via the tube. In this case, when a flexible and thin tube is used, the fluid resistance when ink flows in the tube is increased, which may cause an ink discharge failure in which necessary ink may not be sufficiently supplied to maintain the discharge stability of the ink. Especially, in a large-scale apparatus for printing a recording medium having a wide width, the length of the tube becomes longer. As a result, the fluid resistance of the tube is accordingly increased. Similarly, when fast printing is performed and when the ink having high viscosity is discharged, the fluid resistance is also increased. As a result, a failure of supplying ink to the recording head may occur.
To overcome such failure, as Japanese Patent No. 3606282 (Patent Document 1) discloses, there is a conventionally known technique in which a pressure applied to the ink in the ink cartridge is maintained, and a differential pressure valve is disposed on the ink supply upstream side of the recording head, so that the ink is supplied when the negative pressure of the sub tank is greater than a predetermined pressure.
Further, as disclosed in Japanese Patent Application Publication No. 2005-342960 (Patent Document 2), the ink supply pressure is positively controlled by using a pump to feed the ink to the negative pressure chamber where a negative pressure is generated using a spring, the negative pressure room being disposed on the upstream side of the recording head. Further, as disclosed in Japanese Patent Application Publication No. 5-504308 (Patent Document 3), a pump is similarly used to positively control the pressure without providing a negative pressure chamber.
On the other hand, to obtain the negative pressure with a simple configuration, the ink cartridge communicated with air is communicated with the recording head via a tube, and the ink cartridge is simply disposed below the recording head. By doing this, negative pressure can be obtained by the water head difference.
By using this method, more stable negative pressure can be obtained with much simpler configuration when compared with a method in which a pressure is always applied by using a negative pressure associated valve or a method in which the negative pressure chamber is disposed and the pump is used to supply liquid. However, in this method using the water head difference, the pressure loss due to the tube resistance may become a problem
There is a known method of resolving the pressure loss problem in the ink supply system obtaining negative pressure using the water head difference. In this method, for example, as disclosed in Japanese Patent Application Publication No. 2004-351845 (Patent Document 4), a pump is provided in the tube between the recording head and the ink cartridge and a bypass flow path connecting the upstream side and the downstream side of the pump is provided. Further, a valve is provided in the bypass flow path, and the opening of the valve is appropriately controlled depending on the printing state, so that a desired pressure can be maintained.
However, in the method disclosed in Patent Document 1, the problem of shortage of refill supplies as described above may be resolved. However, the mechanism of controlling the negative pressure is complicated and the demand for the sealing characteristics of the negative pressure associated valve is very high. In addition, the pressure is always required to be applied. Because of this feature, the demand for the sealing characteristics of all the connecting sections in the ink supply flow path is high, and in case of trouble, ink may spout out.
In the method disclosed in Patent Documents 2 and 3, the pump is used to positively control the pressure. Therefore, it is required to accurately control the liquid feeding flow rate by using the pump in response to the consumption flow rate of ink and the like. To that end, for example, it may become necessary to perform a feedback control using the pressure of the negative pressure chamber. Further, for example, when this method is applied to an image forming apparatus using a plurality of different color ink, it is required to separately control the pump for each color ink. As a result, the control may become complicated and the size of the apparatus may be increased.
Also in the method disclosed in Patent Document 4, when this method is applied to an image forming apparatus using a plurality of different color ink, it is required to control the pumps for the respective color inks. As a result, the size of the apparatus may be increased.